Many gas streams used in industrial processes contain so much water vapor that the gas streams must be dried prior to use. Drying of the process gases is required to prevent water from condensing in process equipment, to prevent freeze out in heat exchangers of cryogenic systems and to eliminate water which acts as a contaminant which can adversely affect the process for which the gas is to be used. For example, nitrogen is used as an inerting gas in many chemical processes and as a protective atmosphere during the manufacture of various electronic devices. The presence of water in the protective gas can have a disastrous effect on product quality. Moreover, reactant gases such as methane and hydrogen often must be dried before they can be used in chemical processes.
Typical processes for removing water from gaseous streams employ an adsorbent or desiccant and are capable of removing water vapor to very low levels, for example to a dew point of less than -50.degree. F. However, such processes possess a drawback in that the adsorbent bed must be regenerated, usually by heating and purging the bed with a dry waste gas or using some portion of the product stream if a suitable waste gas stream is not available. Consequently, these systems operate in a cyclic manner requiring duplication of equipment, operation of automated, timed switching valves and separate heater devices. An unavoidable loss of the gaseous feed often occurs during regeneration of the adsorbent.
Several improved processes have been disclosed which require a specific membrane material or membrane configuration (for example, a hollow fiber membrane) permitting preferential permeation of water vapor from the gas to be dried. A universal drawback of such systems is that a significant amount of the feed gas also permeates through the separation membrane and is lost from the gas supply.
Representative processes include U.S. Pat. No. 4,675,030 which describes a system for purifying helium used in a lighter-than-air ship in order to remove air, water vapor and carbon dioxide which has permeated the balloon and contaminated the helium, thus reducing its lifting capacity. A stream of contaminated helium is withdrawn from the balloon, compressed and cooled to condense water which is then separated as liquid and removed. The helium thus dried is reduced in pressure and/or heat exchanged with hot compressed helium to a temperature well above its dew point and then passed to a helium-permeable membrane unit. Pure helium permeating the membrane is returned to the balloon such that mixing with contaminated helium is minimized while impermeate is passed to another membrane unit like the first. Pemeate from the second unit is recycled to the compressor and impermeate gases are vented. According to the data given in this patent, some helium remains in the vent stream and is consequently lost.
U.S. Pat. No. 4,718,921 describes a process for drying a feed gas, such as methane, containing water vapor by passing the gas through a hollow fiber membrane unit in which the membrane material has a permeation rate for water vapor 200 or more times the rate for the feed gas. A portion of the feed gas and most of the water vapor permeates the membrane and is vented and the water-depleted nonpermeating gas is recovered. Enhanced separation efficiency can be accomplished by passing a drying gas, such as argon, containing less than 300 ppm water vapor, along the downstream side of the membrane. A portion of recovered feed gas may be used as the drying gas, but in the examples included to illustrate this configuration, the nonpermeating feed gas did not satisfy the criterion of less than 300 ppm water vapor for the drying gas.
U.S. Pat. No. 4,952,219 describes a low temperature process for separating a gaseous stream into its respective components, such as in the manufacture of nitrogen, in which the feed gas is predried, first by passage through a membrane unit selectively permeable to water vapor and secondly by contact with a molecular sieve to remove water remaining in the nonpermeate gas stream from the membrane unit. Dried waste gas from the low temperature separation can be used as a sweep gas on the downstream side of the membrane. Thus, any of the gas which permeates the membrane is lost from the system. This is of no consequence if the feed gas is, for example, air which can be replaced from the atmosphere, but if the feed gas is a high purity material which is used to create a protective atmosphere in sensitive manufacturing operations, or is a reactant in a chemical processes, the loss of such gases in a drying operation becomes a significant economic disadvantage which must be addressed before such processes can be carried out on an industrial scale.
U.S. Pat. No. 5,067,971 discloses a process for dehydrating gases using composite permeable membranes, preferably hollow fiber composite membranes, comprised of a porous support coated with an ultrathin layer of a defined sulfonated polysulfone or sulfonated polyether ketone. The process also contemplates using a sweep or purge gas on the water-enriched permeate side of the composite membrane to increase efficiency of the dehydration process.